Combustion apparatus and ignitor employing vaporized fuel



Jan} 1960 E. c. ROBERSON ET AL 2,922,279

COMBUSTION APPARATUS AND IGNITOR EMPLOYING VAPORIZED FUEL Filed Jan. 25, 1957 Inve ntorf V Edwin Cecil Roberson, Barnes,

COMBUSTION APPARATUS AND IGNITOR EMPLOYING VAPORIZ E D FUEL London, and Cecil Rounthwaite,-Solihull, England, ass'ignors toPower Jets (Research and, Development) Limited, London, England, a company of Great Britain Application January 25, 1957, Serial No. 636,417

' Claims priority, application Great Britain February 2, 1956 2 Claims. 1 (Cl. 60-39.71)

This invention relates to combustion apparatus particularlythough'not exclusively for use in a gas turbine "designed to operate on heavy distillate fuels.

' Accordingly the invention provides combustion apparatuscomprising a porous block, a fuel supply for satu rating the block with fuel, and an igniter for igniting the fuel emitted from the block as *a vapor. I

In one form of the inventionprovision is made for heating the block to initiate emission of the fuel vapor therefrom. a

According-to a feature of the invention the block is arranged so that the ignited fuel vapor is carried by an air stream into the combustion zone of the apparatus.

The apparatus may further comprise a vaporizing tube projecting at one end into the combustion zone and arranged tobe heated-by the ignited fuel vapor.

The invention also provides an improved construction of a vaporizing tube having a 'cap fitting around its projecting end and forming therewith an annular passage for leading fuel/ air mixture in an upstream direction into the combustion zone. a

One embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is an axial cross-section of a combustion chamber'for use in a'gas turbine plant. The fplantfor which it is intended includes a compressor supplying air to the combustion chamber through a'heat exchanger and a turbine driving the compressor and also supplying useful power, the turbine receiving combustion gases from the combustion chamber and discharging them through the heat exchanger in heat exchange relationship with the incoming air to the combustion chamber. The combustion chamber is intended to operate on diesel or other heavy distillate fuel oil.

The combustion chamber includes an elongated generally cylindrical sheet metal air casing or duct 1 having flanges 2, 3 attached, e.g., by welding, to its upstream and downstream ends for connection to ducting leading from the heat exchanger and to the turbine respectively. Mounted coaxially within the air casing 1 there is an elongated generally cylindrical flame tube 4 with its upstream end spaced downstream from the upstream end of the air casing, the flame tube being supported by an approximately Z-sectioned ring 5 at its downstream end and by three radial pins 6 secured to the air casing 1 and slidably engaging in bushes 7 let into the flame tube wall to permit free radial expansion.

A tubular member 8 coaxial with and of smaller-diameter than the flame tube 4 extends from the upstream end of the air casing 1 to the region of the upstream and of the flame tube 4. This member includes a frustoconical part 8a at its downstream end having a peripheral flange 8b entering into the upstream end of the flame tube and defining therewith an annular gap. The

2,922,279 Patented Jan. 26,1969

gap is maintainedby a crimped spacer 9 which also serves to supp'ort'the downstream endof tubular member 8 from the flame tube 4. The upstream end 'of the tubular member 8.is supported by flexible struts '10 secured at their outer ends to the flange 2.

An'open-ended vaporizing tube 11 is mounted Within member 8 with its downstream end projecting into the flame tube, the tube being supported by a frustoconical wall-12' bolted to the t'ubu'lar member s, the wall being formed with apertures 12a. The tube is formed with a convergent inlet end 11a leading to a construction 11b, and a cap 13 of slightly greater diameter than the tube encloses its downstreamend, the base of the cap being supported therefrom on three pins 'll c. The rim 13a of the cap is of divergent conical shape, while an annular plate 14 is welded to the exterior of the vaporizing tube 11 and partly blocks the annular gap'between the tube and the cap. 1

A main fuel supply pipe'15 consisting of ahyp'odermic tube leads from the exterior of the air casing 1 to an outlet 16 at its end arranged-to inject fuel in a downstream direction in the region of the'constriction 11b in the vaporizing tube. and cooling air tapped from the outlet of the compressor, i.e., upstream of the heat exchanger, is supplied'to the interior of the jacket, finally escaping through holes 17a. This air serves toprotect'the fuel in'pipe '15 against overheating by the heated inlet air to the combustion chamber. g V a I The upstream end of the tubular member 8 is-closed by a transverse'wall 18 pierced with a number-of small holes 18d. Immediately on the downstream side of this wall and withinthe tubular member there is an annular porous block 19 made of two sleeves 19a, 19b of porous ceramic material nested one within theother with an electric heating element 20 accommodated in an an- 'nular space between them. The heating element 20 is connected at 'one end to an electric supply through a terminal 21 in the aircasing wall, the other end being grounded to the tubularmember 8 at 22. A fuel pipe 23 leads intothe interior of the block 19 and immediately, downstream of the block is mounted a sparking plug 24. a

Whenthe combustion chamber is in operation, of the air from the compressor enters the-tubular her 8 through holes 8;- between the block -19 "upstream end of the vaporizing tube 11;-a small of air'also enters throughthe'holes'lSa in the wall 18. This'part ofithe air is divided, part enteringtheupstrearn end of the vaporizing tube and part flowing around the vaporizing tube and entering the flame tube through the apertures 12a in the wall 12. The annular space within the upstream end of the flame tube around the vaporizing tube constitutes the primary combustion zone of the flame tube.

Fuel is injected through outlet 16 into the region of high air velocity afforded by the constriction 11b and is vaporized in the vaporizing tube by the heat of combustion in the flame tube. The flow of air and vaporizing fuel is reversed by the cap 13 which directs it into the primary combustion zone.

The remainder of the air which does not enter the tubular member 8 flows around the exterior thereof and enters the flame tube 4 through the annular gap at its upstream end and through further apertures 4a and 4b in its wall and constitutes cooling, secondary, and dilution air in known manner.

For starting, pilot fuel is also supplied through pipe 23 to the interior of the block 19 so that the latter becomes saturated with fuel. Current is supplied to the heating element 20 so that the fuel is emitted therefrom as a vapor which is ignited by the sparking plug 24.

part

mem- -and the 'quantity The pipe is enclosed by a'jacket 17,

The air stream entering the tubular member 8 through the. holes 18a in the wall 18 and through the holes 80 carries the burning gases into and around the vaporizing tube 11, thus raising its temperature to a value suflicient for vaporization of the fuel introduced by the main fuel injector 16. 'The fuel vapor from this nozzle does not however ignite until it discharges into the primary combustion zone since the fuel/ air mixture in the vaporizing tube is arranged to be too rich to-burn. Ignition in the primary zone is effected by some of the flame from the block passing into the flame tube through the apertures 12a in the wall 12.

When combustion of the main fuel supply has been initiated, the supply of fuel to the block 19 can be discontinued. The block will then dry out and combustion of fuel vapor emitted from its surface will cease.

In normal operation, combustion of fuel/air mixture takes place in the flame tube after it has been discharged from the vaporizing tube and cap. If however the fuel supply is suddenly reduced, e.g. to reduce the output of the plant, the fuel/air ratio in the flame tube will momentarily become much weaker than it is under steady working conditions. As a consequence however, the fuel/air mixture being discharged from the vaporizing tube, which was previously too rich to burn, becomes ignitable and hence the flame burns back and is stabilized in the annular space between the divergent part 13a of the cap and the vaporizing tube. The baffle 14 prevents the flame burning back into the cylindrical part of the cap and thence into the vaporizing tube so that the risk of burning out the vaporizing tube is reduced or eliminated. On richening the mixture again, the flame moves out of the cap and is stabilized in the primary combustion zone in the flame tube.

The mixture discharged around the edge of the baflie flows more or less along the inner surface of the divergent part 13a of the cap, thus cooling it and maintaining it at a safe operating temperature even under full load conditions.

In some cases the compressed air supplied to the combustion chamber may be at a sufficiently high temperature to vaporize the fuel in the block; the electric heating element 20 may then be omitted.

The apparatus described may also be used for the burning of lighter distillate fuels, e.g., kerosene, in which case the heating element 20 in the porous block may not be required.

The vaporizing tube described above might be replaced by any known vaporizing device of a similar nature.

We claim:

1. Combustion apparatus of the vaporizing type designed to operate on heavy distillate fuels comprising an elongated air casing, said air casing having an inlet at one end for combustion air and an outlet at the other for combustion gases; a porous block mounted adjacent said inlet; a first fuel supply connected to said block to saturate the block with fuel; a heating element for heating the block; an ignitor arranged to ignite fuel emitted from the block as a vapor; means defining a combustion zone within the casing down stream of the block, a vaporizing tube; said vaporizing tube being supported in the casingto extend for part of its length within the combustion zone and having one open end arranged to receive ignited vapor from the block and another end within the combustion zone; a second fuel supply; a fuel injector arranged to inject fuel from said second fuel supply into said vaporizing tube whereby fuel injected by said fuel injector is vaporized and ignited, on demand by ignited fuel vapor emitted from said block and carried downstream into said means defining a combustion zone.

2. Combustion apparatus of the vaporizing type designed to operate on heavy distillate fuels comprising an elongated casing having an inlet for air at one, upstream, end and an outlet for combustion gases at its other downstream end', a combustion initiating device operable on demand supported adjacent the inlet end of said casing; a flame tube mounted within the casing; ber arranged to intercommunicate said inlet with said flame tube; a vaporizing tube, a perforate annular member adapted to' support said vaporizing tube within the tubular member with a part of its length extending within the flame tube; said vaporizing tube having an outlet end within the flame tube and an inlet end within the tubular member; a first fuel supply; a fuel injector arranged to introduce fuel from said first fuel supply into the vaporizing tube, said combustion initiating device including; a porous block mounted within the tubular member; a second fuel supply for saturating the block with fuel; means for heating the block and an ignitor for igniting fuel emitted from said block as a vapor, whereby ignited vapor can pass downstream from said inlet through and around the vaporizing tube into the flame tube to preheat fuel from said first supply injected into the vaporizing tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,628,475 Heath Feb. 17, 1953 2,712,352 Manor et al. July 5, 1955 2,720,753 Sharpe Oct. 18, 1955 FOREIGN PATENTS 712,843 Great Britain Aug. 4, 1954 a tubular mem-v 

